The present invention is generally directed to apparatus and related methods for providing highly maneuverable shape memory alloy devices. More particularly, the present invention relates to devices with separately addressable thin-film shape memory alloy actuators.
Shape memory alloys are a unique group of materials that exhibit memory retentive properties. A shape memory alloy element may be trained with a high temperature shape, and may also have a relatively deformable low temperature shape. Changes in surrounding temperatures result in a phase transformation in its crystalline structure. At lower temperatures, shape memory alloys are relatively deformable and exist in what is known as a martensitic phase. Meanwhile, at higher temperatures, these materials experience a phase transformation towards an austenitic phase which is more rigid and inflexible. The temperature at which the phase transition occurs is referred to as the activation temperature. A shape memory alloy element may be initially imprinted or trained with a particular configuration when heated to a temperature much higher than the transition temperature. Shape memory alloys have been observed to repeatedly recover their memory shape when heated above their respective transition temperatures very rapidly, and with great constant force over a wide range of retentive strain energy. The ability of shape memory alloys to remember their high temperature trained shape makes them particularly suitable for actuating devices that provide useful work and directional movement.
The basis for selecting shape memory alloys in the construction of conventional steerable elements such as a flexible catheters is primarily their ability to reversibly change shapes during their microstructural transformation. At lower temperatures, shape memory alloys are relatively soft and may exhibit a Young""s modulus of approximately 3000 MPa. In a martensitic phase, the shape memory alloy may be readily deformed up to about 5% in any direction without adversely affecting its memory properties. When heated just beyond its activation temperature, the transformation process commences, and the material becomes a harder, inflexible material that may have a Young""s modulus of approximately 6900 MPa in an austenite or parent phase. When the shape memory alloy material is not excessively deformed or constrained, it attempts to reorganize its structure to a previously trained or memorized shape. Upon cooling, the shape memory alloy again, becomes soft and may be mechanically deformed to begin another cycle. The mechanical deflections produced by activating the memorized state can produce useful work if suitably configured in apparatus such as actuation devices. Although the measurable recovery deflections may be relatively small, the recovery forces and energy have been observed to be extremely high and constant.
A common example of a shape memory alloy includes nickel titanium alloys (NiTi), also known as nitinol, which may vary in relative percentages of composition. The activation temperature of a particular shape memory alloy may be changed according to its elemental composition. When the alloy is heated through its transformation temperature, it reverts back to its austenite phase, and recovers its shape with great force. The temperature at which the material remembers its high temperature form may be adjusted by changes in alloy composition and specific heat treatment. For example, activation temperatures for NiTi alloys may be readily altered from 100xc2x0 C. above or below zero. The shape recovery process, however, may be controlled and occur over a range of just a few degrees or less if necessary. A wide variety of shapes may be programmed into an shape memory alloy actuator element by physically constraining the piece while heating it to an appropriate annealing temperature. NiTi is commercially available in sheet, tube and wire forms, and may have a wide range of transformation temperatures. The memory transformation of an shape memory alloy element is dependent upon temperature. However, the rate of deformation is largely dependent upon the rate of cooling and heating. The rate at which temperature changes take place often dictates the relative speed at which the actuator can operate. A faster actuating shape memory alloy actuator must often be heated and cooled more readily, and has been known to consume more power and generate an excess amount of dissipated heat.
Shape memory alloy actuators have been used in numerous steerable devices such as catheters. These devices are limited in dexterity, however, and movement is often limited to a single plane and not in a rotational direction. Shape memory alloy elements must also be mechanically deformed to begin another cycle. Each shape memory element is often coupled to a biasing element or at least one other shape memory element. When one of the elements is heated and moves towards its predetermined shape, it is returned to an original position or shape by the biasing element or the activation of another memory element. This generally enables controlled motion but only in a single plane, and may provide only up to two degrees of freedom. Moreover, the relative dimensions of actuator joints are often excessively large and cumbersome since an opposite force is needed to return the shape memory alloy element to its initial martensitic shape. In general, complex linkages are also required to rotate these steerable devices. The range of maneuverability is severely limited by the linkages which are necessary to return the element to its martensitic shape after it has been activated and cooled. Conventional steerable devices using shape memory alloys are also relatively large and have a severely constrained lower size limit. The relatively large size of present actuators is mainly attributed to sizeable control arms, linkages or other elements needed to return the shape memory actuator to its initial state. This severely constrains the geometry of such a conventional steerable device. Available shape memory alloy devices today also lack the precise control necessary to maneuver into very small, geometrically complex spaces. Moreover, current actuators are often too slow for many medical applications where quick, dexterous movement is required. Large steerable devices with shape memory alloy elements often require an increased amount of current in order to produce the activation temperature needed for a quick transition from the martensitic state to the programmed or memorized austenitic phase. A conventional shape memory alloy actuator consumes a great deal of power, thus dissipating a large amount of heat. This necessarily slows down the cooling to the activation threshold, and slows down the transition from the austenitic state back to the martensitic state resulting in a slower acting device.
There is a need for an efficient actuator device that is capable of unrestricted yet highly precise and dexterous maneuvers in three-dimensional space. It would be advantageous to reduce the need for control arms, linkages, or other extraneous mechanical devices for returning conventional shape memory alloy elements to a first position after deactivation, and their transition from the parent phase back to the martensitic state. There is a further need for shape memory alloy actuators that provide unrestricted linear and rotational movement. These devices should be saleable to provide increased dexterity and maneuverability in very small, geometrically constrained areas which are presently inaccessible by conventional steerable devices. An effective heating system is further required to activate highly detailed actuator patterns formed from shape memory alloys or any other material with memory capability. It would further desirable to form a variety of actuator arrays from a minimum number of shape memory alloy sheets to simplify the production and the assembly process.
The present invention provides shape memory alloy actuator apparatus and related processing methods. An object of the invention is to provide shape memory alloy apparatus with a full range of linear and rotational movement with variable stiffness.
In one embodiment of the invention, a shape memory alloy actuator array is formed from a plurality of individually trained shape memory alloy actuators to provide relative movement of different array portions, and a thin-film heating element positioned adjacent at least one shape memory alloy actuator to thermally activate the actuator for movement away from its initial shape. The shape memory alloy actuators m,ay be positioned in between at least two connecting rings and adjacent another shape memory alloy actuator along a different portion of the connecting rings. The actuators may be further positioned in side by side pairs with a biasing element for returning the actuator to its initial shape. The side by side pairs may be formed along the periphery of the connection rings and include one actuator that expands or extends towards a predetermined shape when heated and one actuator that contracts towards a predetermined shape when heated. Alternatively, the plurality of shape memory alloy actuators may be positioned to act in opposition to at least one other actuator formed of shape memory alloy, elastomer material or a spring. The total number of shape memory alloy actuators that are trained to expand when activated may be equal to or different than the total number of shape memory alloy actuators that are trained to contract when activated.
An additional object of the present invention is to provide a shape memory alloy array with actuators having initial nonplanar shapes and substantially planar predetermined shapes. The shape memory alloy actuators may also have an initial buckled shape that provides useful work when activated towards its substantially planar predetermined shape. The buckled configuration of the shape memory actuators exploit certain advantages of force amplification to effect relative movement of different array portions.
It is a further object of the present invention to vary the stiffness of a shape memory alloy device through the activation of a combination of at least one actuator.
Another embodiment of the present invention provides a shape memory alloy catheter comprising a catheter body formed with a sidewall portion, a shape memory alloy portion positioned adjacent the catheter sidewall portion having a lattice network of individually configured shape memory alloy micro-actuators, and an addressable thin-film heater element in communication with the shape memory alloy portion for activation of selected micro-actuators. More particularly, the shape memory alloy catheter may further include connecting rings or intermediary spacers for separating the device into segmented joints with at least one micro-actuator that expands upon heating by an addressable heater element, and at least one micro-actuator that contracts upon heating by another addressable heater element. A selected combination of at least one micro-actuator may be activated for varying the relative stiffness of the shape memory alloy portion.
Another variation of the present invention is directed to a shape memory alloy conduit comprising a lattice structure formed of shape memory alloy micro-actuators, and a network of heating elements formed about the lattice structure for activating selected shape memory actuators within the lattice structure. The network of heating elements activates a selected combination of at least one actuator in the conduit which may provide relative movement between conduit portions, or vary the relative stiffness of lattice structure portions. The lattice structure may include connecting rings with intermediary shape memory alloy micro-actuators that may expand or contract when heated. The network of heating elements may be thin-film addressable heating elements controlled by a microprocessor unit that selectively activates a combination of at least one micro-actuator for relative movement of the shape memory alloy conduit or for variable stiffness.
In yet another embodiment of the present invention, a shape memory alloy apparatus and associated methods provide a shape memory alloy medical device comprising a scaffolding formed of individually activated and oppositely trained shape memory alloy actuators set with a predetermined shape to provide a full range of directional movement within a body, and at least one heating element in communication with the scaffolding surface to selectively activate a combination of at least one actuator towards a predetermined state. The scaffolding may include at least two connecting rings to support relative movement of the shape memory alloy medical device. The actuators within the scaffolding may have substantially rectangular configuration with a buckled surface longitudinally and laterally aligned relative to the scaffolding. It is a further object of the invention to provide a system of separately addressable thin-film heaters that thermally activates a selected combination of at least one actuator to vary the ring to ring tilt or rotational angle of the scaffolding within a predetermined range. The plurality of heating elements may also thermally activate a selected combination of at least one trained actuator towards an intermediate state for variable stiffness and relative movement of the device within the body.
It is another object of this invention to provide a directional actuator device comprising a skeletal structure formed of oppositely trained shape memory alloy actuators each configured with a predetermined shape, and a heating system having individual localized heaters for moving each actuator towards its predetermined shape. The skeletal structure may further include a backbone and a shape memory alloy portion that contracts when thermally activated, and a shape memory alloy portion that expands when thermally activated to provide for arcuate movement of the actuator device. The skeletal structure may be further formed with a supporting ribbed cage section. At least a portion of the directional actuator may be encapsulated within at least one polymeric coating.
Another embodiment of the present invention includes a thermally activated directional actuator device with a skeletal structure and a plurality of intermediary spacers or connecting rings for supporting relative movement of the directional actuator portions. The intermediary spacers may further include actuator extensions for connection to actuators. The skeletal structure may be formed with at least two oblong actuators longitudinally aligned relative to the structure and at least two oblong actuators laterally aligned relative to the structure for relative movement of the skeletal structure portion. The connecting rings may be formed with actuator extensions for connecting actuators laterally aligned relative to the actuator device. The laterally aligned actuators may also include at least one actuator that expands in length when heated and at least one actuator that contracts in length when heated.
It is a further object of the present invention to provide a method of forming a shape memory alloy actuator device comprising the following steps of: selecting a sheet of shape memory alloy material defined by at least two side edges; forming a plurality of shape memory alloy actuators to provide relative movement of the actuator by removing selected window portions of the sheet along a series of spaced apart rows and columns; individually training the shape memory alloy actuators to a predetermined state; laying out a thin-film network of addressable heating elements onto the sheet for selectively activating the shape memory alloy actuators; and sealing the side edges of the sheet to form a shape memory alloy actuator array. The spaced apart rows may form connecting rings to support relative movement of the shape memory alloy actuator array. The spaced apart columns may generally define the lateral portions of the shape memory alloy actuators. The plurality of shape memory alloy actuators may also be formed in side by side pairs. Each trained shape memory alloy actuator may move towards a predetermined shape by heating, and may be trained to expand or to contract when activated. The network of addressable heating elements may also be connected to a microprocessor unit for selectively activating a combination of at least one shape memory alloy actuator.
In yet another embodiment of the present invention, an additional thin-film sheet of shape memory alloy material may be selected to provide for an actuator formed of multiple sheets. The shape memory alloy actuators formed in the first thin-film sheet may be trained to expand when heated, and the shape memory alloy actuators formed in the second thin-film sheet may be trained to contract when heated. These and other objects and advantages of the present invention will become more apparent from the following description and accompanying drawings.